The tone of this poem is sad because Robert Frost is talking about the world ending, and things perishing, and sufficing things. He is talking about people talking about the world ending in fire and ice, and people favoring fire, and destruction of ice, but he is also talking about something being great.
The theme of the story is to show how people say the world will end by people being mean and nice people too and saying things are great and things like that.
Ice is a symbol because it symbolizes cruel/cold people in this world, fire symbolizes nice/warm hearted people in the world.